Perhaps a very high speed finishing cut would sort it

(or a finishing cut on a bridgeport !)

I get the same effect on my home made mill but it's not very stiff/heavy

Edited By Ady1 on 13/02/2023 23:28:36

Are you conventional milling, against the direction of rotation of the cutter, or climb milling in the same direction as the rotation?

Do you have all slideways locked except the one needed to allow the motion of feed?

What size cutters are you using?

RPM, locking up unused ways and machine rigidity are not the only thing that matter. In order to cut there needs to be a big enough bite into the workpiece from the cutting edge. Too small and it will rub the surface, too big and it will damage the cutter. To get a good finish requires a steady motion at a suitable speed. Anywhere where the tool is paused will be rubbed below the surface that can be cut. This rubbing is not good for either the finish or the cutting edge of the tool.

When someone refers to speeds and feeds these are the two factors that are being talked about. Speed is the rate at which the cutting edge is passing over the surface it is cutting and is calculated from the RPM and the diameter of the cutter. Feed is how fast the tool axis and workpiece are moving relative to each other. Feed is used to obtain a sensible cut each time the cutting edge engages with the workpiece. For example, if you have a single cutting edge on a tool rotating at 60 rpm and travelling 3mm per minute the tool will rotate 60 times in that minute and feed 3mm at the same time. The chip thickness will be 3mm / 60mm/s­ = 0.05mm. Two cutting edges on the tool will halve the chip thickness to 0.025mm.

Doubling the rpm of the single edge tool will also halve the chip thickness from 0.05mm to 0.025mm.

So, going back to the original problem, if you are using a feed rate that is too slow then there may be times when the tool is dwelling in one spot and rubbing. If you are using powered feed that is too slow the tool can go from cutting to rubbing to cutting. The chips that are coming off should give some indication of correct feed, you do not want very fine slivers, they should be discrete chips.

Another factor is tool flex. How much stick-out is there? A long thin tool will deflect much more than a short fat tool. A 3 or 4 flute cutter is more rigid than a 2 flute tool. Adaptors that extend the tool out from the spindle (R8 to ER for example) can increase the stick-out as well.

There is one other factor that can cause alternation between cutting and rubbing and that is the state of the bearings of the spindle. If you have a relatively new machine it is possible that the bearings have started to settle in place and may need adjusting to take up some slack. An older machine needs adjustment if the bearings start to wear. A simple test for correctly adjusted bearings is that they should get warm but not hot when working. So the question is, if you run the spindle at full rpm for about 15 minutes but doing no work with the machine do the bearings at the top and bottom of the spindle get warm to the touch, stay cold or get too hot to touch. Warm is what you want. Frequent need to adjust the bearings means they need replacing.

Martin C

Edited By Martin Connelly on 14/02/2023 08:58:40

I always climb mill the last thou or two, it produces a much better finish.

Don’t see any mention of how sharp the cutters are, if they are carbide or HSS, a decent brand or cheap rubbish ones also the OP says he is using the correct rpm for cutter size, where is this information gleaned from, big difference between model making feeds and speeds and those that are used in industry, how true is the cutter running, exactly how rippled is the surface, maybe a photo will assist us, maybe it’s just a standard finish from the side of a cutter, how experienced is the OP,